FIG. 6 illustrates a conventional photoelectric switch in which reference numeral 1 is a pulse generating circuit; 2 is a light emitting diode driving circuit (hereinafter called the LED driving circuit) which comprises a series circuit of a light emitting diode 23 (hereinafter called the LED), a transistor 21 and a resistor 22, and series connected resistors 24 and 25 coupled to the output end of the pulse generating circuit 1. The base of transistor 21 is connected to the point between the resistors 24 and 25. Reference numeral 3 is a light receiving element, and reference numeral 4 is a light receiving circuit. Reference numeral 5 is an output circuit that includes a Zener diode 51, a resistor 52 and an output transistor 53. The base of the output transistor 53 is connected to a series circuit that includes a transistor 54 driven by the output of the light receiving circuit 4. Reference numeral 6 is a constant voltage power supply circuit and reference numeral 7 is a smoothing capacitor. Reference numeral 10 is a power supply circuit connected in series with the load 11 to the output terminals (T1 and T2) of the photoelectric switch.
In the conventional photoelectric switch of FIG. 6, when the power supply 10 and the load 11 are connected to the output terminals (T1 and T2) the smoothing capacitor 7 is charged to a predetermined voltage via the constant voltage power supply 6. When a pulse voltage is output from the pulse generating circuit 1, the transistor 21 is activated, and a current flows from the capacitor 7 to the LED 23. When the current flows through the LED 23, a pulse light is emitted by the LED 23. The light receiving element 3 receives the pulse light from the LED 23 and converts it to an electric signal. The output of the light receiving element 3 is amplified and detected by the light receiving circuit 4, and is output as a detected output in accordance with the intensity of the light received. When the detected output is output from the light receiving circuit 4, the transistor 54 is activated. A base current flows into the output transistor 53 via the transistor 54 and the Zener diode 51 activating the transistor 53 and driving the load 11. At this time, the voltage across the output transistor 53 is nearly the sum of the Zener voltage V.sub.z of the Zener diode and the drop in the base emitter voltage V.sub.bc across the output transistor 53.
The light projecting circuit of the conventional photoelectric switch described above requires a high voltage for the LED driving circuit due to the large voltage drop across the LED 23. In addition, the power supply voltage must be large since there is a large voltage drop between the output terminals (T1 and T2) when the photoelectric switch is turned on. This reduces the voltage applied to the load, resulting in a deficiency.